In the telecommunication art, the term "crosstalk" refers to interference that enters a message channel from one or more other channels through a path coupling the message channel with the interfering channels. Crosstalk can create annoyance in a voice system or errors in a data system. Crosstalk's impact depends upon such factors as the listener's hearing acuity, the extraneous noise, the frequency response characteristic of the coupling path, and the level of the disturbing signal.
In wire cable technology, coupling between wire pairs--the signal propagation medium--occurs because of imperfections in the physical composition of the cable itself. Energy on one wire pair is electromagnetically coupled to the other wires due to the unbalances caused by the imperfections. This induced coupling has several manifestations, including: the coupling varies randomly from cable-to-cable and even from wire pair-to-wire pair in a given cable; and the coupling depends on the terminating impedances of the wire pairs and on the total length of exposure, i.e., electrical distance of the wire pairs.
There are generally two types of crosstalk mechanisms that are characterized, one being the near-end crosstalk (NEXT) and the other far-end crosstalk (FEXT). NEXT results from a distrubing source connected at one end of the wire pair causing interference in the message channel at the same end as the disturbing source. FEXT is that portion of the disturbing signal propagated to far end of the message channel, that is, the end opposite to the end to which the disturbing source is connected.
In full-duplex loop transmission systems, NEXT compatibility is the dominant concern to ensure proper performance of a communication system, and this is especially true today since a multitude of systems with different spectral characteristics compete to provide integrated data-voice services. In one conventional measurement approach, NEXT is evaluated using computer simulations of the crosstalk impairment to estimate the the NEXT margin in a variety of scenarios. The NEXT margin in a digital system refers generally to the increase in level of the interfering source, relative to a predetermined reference condition, that causes a preselected error rate. The margin provides a measure of the "robustness" of performance for a given system when exposed to crosstalk from another system. Given the right technical information about the systems under study, such estimates can be quite accurate. In fact, because the NEXT model inherently contains information from a statistically meaningful sample of crosstalk paths, an analytical model of the impairment is possibly more valuable than appropriate measurements of the actual NEXT margin based on a statistically insignificant sample.
However, even the best computer simulations benefit from experimental verification. Before a simulation can be used to explore the performance of a system, there must be a high degree of assurance that the simulation accurately characterizes the system. Accordingly, measurement of NEXT margins between loop transmission systems is a fundamental component to support the spectrum management process. Unfortunately, current conventional techniques for achieving empirical verification are awkward and expensive. Even the best method currently available falls short of satisfying the levels of speed and flexibility required when the analysis of a large number of systems with different signal formats is contemplated.